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Summing UVOT images

You may want to sum images to detect faint sources. To sum extensions within an image use uvotimsum. If you wish to sum images from different observations, the separate FITS files should be merged first, using fappend.

Check that all your images are correctly aligned to the World Co-ordinate System: look in the image header for the keyword ASPCORR. If ASPCORR = 'DIRECT' then your images are already correctly aligned. If not, run the task uvotskycorr TWICE (first to ID the sources and then use that output correction file to WCS-correct the image). You will need a star catalogue file (get usnob1.spec here), and the spacecraft attitude file *sat.fits from the auxil directory of your observation.

Assuming the ASPCORR keyword is not set to DIRECT, here are the steps to perform the aspect correction:

>uvotskycorr catspec=usnob1.spec
What to do (ID|SKY) [ ] ID
Input SKY image file name(s)[ ] sw00374210000uuu_sk.img.gz
Input corrections file[ ] NONE 
Input attitude file[ ] sw00374210000sat.fits.gz
Output file name[ ] Ucorr.fits

>uvotskycorr catspec=usnob1.spec
What to do (ID|SKY) [ ] SKY
Input SKY image file name(s)[ ] sw00374210000uuu_sk.img.gz
Input corrections file[ ] Ucorr.fits
Input attitude file[ ] sw00374210000sat.fits.gz
Output file name[ ] 

Once the aspect correction has been performed on all the images and extensions, they can be summed as required:

>uvotimsum
Name of input image file [ ] sw00374210000uuu_sk.img.gz
 Output file name [ ] Usum.fits

or, if multiple Obs IDs are required (note the image must now be uncompressed):

>cp Usum.fits Usumall.fits
>fappend 001sum.fits Usum_all.fits

where 001sum.fits was created using uvotimsum on the segment 001 data (sw00374210001uuu_sk.img.gz). As many images as required can be added together in this manner. Use uvotimsum exclude=NONE when summing the extensions in Usum_all.fits, since the aspect correction keyword is not copied when uvotimsum is run. uvotdetect can be run on this combined image (Usum_all.fits) to determine whether the source has been detected.

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